Polymer membrane for a fuel cell, a method of preparing the same, and a membrane-electrode assembly fuel cell system comprising the same

ABSTRACT

A polymer electrolyte membrane for a fuel cell, a method of preparing the same, and a fuel cell system comprising the same. The polymer electrolyte membrane includes a metal-bound inorganic ion-conductive salt and an ion-conductive cation exchange resin.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to and the benefit of Korean PatentApplication No. 10-2005-0054955 filed in the Korean IntellectualProperty Office on Jun. 24, 2005, the entire content of which isincorporated herein by reference.

FIELD OF THE INVENTION

The invention relates to a polymer electrolyte membrane for a fuel cell,a method of preparing the same, and a fuel cell system comprising thesame. More particularly, the invention relates to a polymer electrolytemembrane for a fuel cell, which is capable of operating at hightemperatures, a method of preparing the same, and a fuel cell systemcomprising the same.

BACKGROUND OF THE INVENTION

A fuel cell is a power generation system for producing electrical energythrough an electrochemical redox reaction of an oxidant and hydrogen ina hydrocarbon-based material such as methanol, ethanol, or natural gas.

Typical examples of fuel cells are polymer electrolyte membrane fuelcells (PEMFC) and direct oxidation fuel cells (DOFC). A direct oxidationfuel cell that uses methanol as a fuel is called a direct methanol fuelcell (DMFC). The polymer electrolyte membrane fuel cell is anenvironmentally-friendly energy source that can replace fossil fuelenergy. It has several advantages such as high power output density,high energy conversion efficiency, operability at room temperature, andthe capability to be down-sized and tightly sealed. Therefore, it can bewidely applied to various areas such as non-polluting automobiles,residential electricity generation systems, and as portable powersources for mobile communication equipment and military equipment.

The polymer electrolyte membrane fuel cell has an advantage of havinghigh energy density, but it also has the problems of requiring carefulhandling of hydrogen gas, or requiring accessory facilities such as afuel-reforming processor for reforming a fuel gas such as methane,methanol, or natural gas to produce the hydrogen required.

In contrast, a direct oxidation fuel cell generally has lower energydensity than that of a polymer electrolyte fuel cell, but it has theadvantages of easy handling of the liquid-type fuel, operability at lowtemperatures, and does not require additional fuel-reforming processors.Therefore, such direct oxidation fuel cells may be appropriate systemsfor small-scale and general purpose portable power sources.

It is also highlighted as a novel portable power source because it hasfrom four to ten times higher energy density than that of small lithiumbatteries.

The fuel cell has a stack formed by stacking several to a plurality ofunit cells in multi-layers, which generates electricity. Here, each unitcell is made up of a membrane-electrode assembly (MEA) and a separator(also referred to as a bipolar plate).

The membrane-electrode assembly has an anode (referred to as a fuelelectrode or an oxidation electrode) and a cathode (referred to as anair electrode or a reduction electrode) separated from each other by apolymer electrolyte membrane.

As for the polymer electrolyte membrane, research on a polystyrenesulfonic acid-based polymer resin has been actively performed since itsinitial development stage in the 1960s. In 1968, E. I. Dupont de Nemors,Inc. developed a perfluorinated sulfonic acid-based cation exchangeresin (product name: NAFION®), which is reported to have much improvedproton conductivity, electrochemical stability, and so on. However,since then, research has been more widely focused on the practical useof a fuel cell using NAFION®. NAFION® has hydrophobicpolytetrafluoroethylene as a main chain and a functional group includinga hydrophilic sulfone group at its side chain. On the other hand, afluorine-based cation exchange resin with a similar structure to NAFION®has been developed by Asahi Chemical, Asahi Glass, Tokuyama Soda, and soon.

However, a NAFION® polymer electrolyte membrane, which has alreadybecome commercially available, has many more advantages than ahydrocarbon-based polymer electrolyte membrane in terms of oxygensolubility, electrochemical stability, durability, and the like. Sincethe NAFION® polymer electrolyte membrane appears to be conductive tohydrogen ions when about 20% of the polymer weight therein becomeshydrated (i.e. a sulfone group included in a pendant group is hydrolyzedinto a sulfonic acid), a reaction gas used in a fuel cell must besaturated by water to hydrate the electrode membrane. However, the watergradually evaporates above its boiling point of 100° C., and accordinglythe resistance of the polymer electrolyte membrane increases,deteriorating cell performance. In addition, the NAFION® polymerelectrolyte membrane, which is commonly 50 to 175 μm thick, can beincreased or decreased in thickness to improve the dimensional stabilityand mechanical properties of a fuel cell. However, when the thickness isincreased, the conductivity of the polymer electrolyte membranedecreases, and when it is decreased, the mechanical propertiesdeteriorate. When the polymer electrolyte membrane is used in a methanolfuel cell, non-reacted liquid methanol fuel passes therethrough duringthe cell operation (i.e., methanol crossover), thereby deterioratingcell performance as well as causing a fuel loss, because the methanol isoxidized at a cathode.

Therefore, various methods for preventing the methanol from crossingthrough the polymer electrolyte membrane have been recently researchedand suggested. For example, a method of sputtering palladium on thesurface of a polymer electrolyte membrane or forming a thin polymerlayer with high resistance against methanol transmission thereon byplasma polymerization has been reported. Another method, of formingnano-sized silica (SiO₂) on the polymer electrolyte membrane in asol-gel method, has also been revealed.

However, the modified polymer electrolyte membranes produced by using asputtering and plasma method are insufficiently competitive in price.The sol-gel method of forming silica also has a problem of lowproductivity since it needs a great deal of washing to prevent silicafrom being poisoned by Cl⁻ ions due to the reaction of the precursor ofsilica (tetraethoxyorthosilicate) with hydrochloric acid.

SUMMARY OF THE INVENTION

One embodiment of the invention provides a polymer electrolyte membranefor a fuel cell which can operate at a high temperature.

Another embodiment of the invention provides a method of preparing thepolymer electrolyte membrane for a fuel cell.

Yet another embodiment of the invention provides a fuel cell systemincluding the polymer electrolyte membrane for a fuel cell.

According to an embodiment of the invention, a polymer electrolytemembrane that includes a metal-bound inorganic ion-conductive salt andan ion-conductive cation exchange resin is provided.

According to another embodiment of the invention, a method of preparingthe polymer electrolyte membrane is provided, which includes preparing ametal-bound inorganic ion-conductive salt by reacting a metal salt andan inorganic ion conductor in a solvent, mixing the inorganicion-conductive salt with an ion-conductive cation polymer resinsolution, and finally preparing the polymer electrolyte membrane bycasting or electrospinning the mixture.

According to one embodiment of the invention, a membrane-electrodeassembly is provided which includes an anode and a cathode facing inopposite directions and the polymer electrolyte membrane positionedtherebetween. In an embodiment, the polymer electrolyte membraneincludes a metal-bound inorganic ion-conductive salt and anion-conductive cation exchange resin.

According to still another embodiment of the invention, a fuel cellsystem is provided that includes at least one electricity generatingelement including at least one membrane-electrode assembly, whichincludes an anode and a cathode positioned to face in oppositedirections and a polymer electrolyte membrane interposed therebetween,and a separator, that generates electricity through the electrochemicalreaction of an oxidant and a fuel, wherein a fuel supplier supplies theelectricity generating element with the fuel and an oxidant suppliersupplies the electricity generating element with the oxidant. In oneembodiment, the polymer electrolyte membrane includes a metal-boundinorganic ion-conductive salt and an ion-conductive cation exchangeresin.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention, and many of the attendantadvantages thereof, will be readily apparent as the same becomes betterunderstood by reference to the following detailed description whenconsidered in conjunction with the accompanying drawings.

FIG. 1 is a schematic diagram showing the structure of a fuel cellsystem prepared according one embodiment of the invention;

FIG. 2 is a SEM photograph of a polymer electrolyte membrane preparedaccording to Comparative Example 2;

FIG. 3 is a SEM photograph of a polymer electrolyte membrane preparedaccording to Example 1 of the invention;

FIG. 4 is a graph illustrating conductivity change of a polymerelectrolyte membrane prepared according to Examples 1 and 2 andComparative Example 1 of the invention as the temperature thereof ischanged;

FIG. 5 is a graph illustrating the methanol cross-over current value ofa polymer electrolyte membrane prepared according to Example 1 andComparative Examples 1 and 2; and

FIG. 6 is a graph illustrating power densities of cells according toExample 1 and Comparative Examples 1 and 2.

DETAILED DESCRIPTION

In one embodiment, the invention relates to a polymer electrolytemembrane for a fuel cell, and particularly to a polymer electrolytemembrane for a fuel cell which is free from the problems of aconventional perfluorosulfonic acid resin polymer electrolyte membrane.That is, even if the conventional membrane has high proton conductivitywhen a sulfonic acid moiety at its side chain is dissociated bymoisture, the cell performance deteriorates as the resistance of themembrane sharply increases, but conductivity decreases due to thedehydration reaction at a temperature over the boiling point, 100° C.,of the moisture.

In addition, an embodiment of the invention relates to a polymerelectrolyte membrane for a fuel cell that can prevent a problem ofdeteriorating cell performance as a hydrocarbon fuel moves toward acathode through the polymer electrolyte membrane and is oxidized at thecathode, decreasing oxidant reduction space at the cathode, i.e.,suppressing the crossover of the hydrocarbon fuel.

In one embodiment, the polymer electrolyte membrane for a fuel cell ofthe invention includes a metal-bound inorganic ion-conductive salt andan ion-conductive cation exchange resin.

In an embodiment, the inorganic ion-conductive salt has a fine powdershape as a hydrophilic inorganic ion conductor, and can prevent hydrogenion conductivity from deteriorating due to evaporating moisture when thepolymer electrolyte membrane operates at a temperature over 100° C.Illustrated in further detail, when the inorganic ion-conductive salt isdispersed into a composition for preparing the polymer electrolytemembrane as a powder with a diameter of about 10 to 500 nm, it canincrease the space in which hydrogen ions and/or moisture can move. Inaddition, since the inorganic ion-conductive salt is not water-soluble,it is not dissolved by the moisture produced during the cell operation.Therefore, it can prevent the crossover of a hydrocarbon fuel, unlike aconventional water-soluble inorganic ion conductor such asphosphotungstic acid, which is dissolved by moisture produced during thecell operation and leaves the polymer electrolyte membrane with themoisture, thereby forming minute pores.

In an embodiment, non-limiting examples of the inorganic ion conductormay include at least one selected from the group consisting ofphosphotungstic acid, silicotungstic acid, zirconium hydrogen phosphate,α-Zr(O_(a1)PCH_(a2)OH)_(a)(O_(b1)PC_(b2)H_(b4)SO_(b5)H)_(b).nH₂O,γ-Zr(PO_(a1))(H_(a2)PO_(a3))_(a)(HO_(b1)PC_(b2)H_(b3)SO_(b4)H)_(b).nH₂O,Zr(O_(a1)PC_(a2)H_(a3))_(a)Y_(b), Zr(O_(a1)PCH_(a2)OH)_(a)Y_(b).nH₂O,α-Zr(O_(a1)PC_(a2)H_(a3)SO_(a4)H)_(a).nH₂O, α-Zr(O_(a1)POH).H₂O, andcombinations thereof. In the above formula, a1, a2, a3, a, b1, b2, b3,b4, b5, and b are the same or different integers ranging from 0 to 14,and n is an integer ranging from 0 to 50.

A heteropolyacid compound bound with the metal works as a moisturecarrier of the ion-conductive cation exchange resin at a hightemperature and can improve the stability of high temperatureconductivity. Consequently, since a fuel cell having a polymerelectrolyte according to one embodiment of the invention can be operatedat a high temperature, it can improve reaction speed. In addition, sinceCO is not easily bound with the catalyst at a high temperature, it canhave an effect on preventing a catalyst from be poisoned by CO.

In one embodiment, the metal can include a univalent metal, for exampleCs, Na, or K.

In another embodiment, a metal-bound inorganic ion-conductive salt ofthe invention includes an inorganic ion conductor with a metal ionsubstituted for a proton ion. In one embodiment, a H⁺-inorganic ionconductor is transferred into a metal⁺-inorganic ion-conductive salt.

In an embodiment, the metal-bound inorganic ion-conductive salt of theinvention is a salt of Cs-bound phosphotungstic acid.

In an embodiment, the ion-conductive cation exchange resin includes apolymer resin containing at its side chain a cation exchange groupselected from the group consisting of sulfonic acid groups, carboxylicacid groups, phosphoric acid groups, phosphonic acid groups, andderivatives thereof, and combinations thereof. The hydrogen ionconductivity of the polymer resin can be adjusted by equivalent weight,which can be obtained from an ion-exchange ratio of the ion exchangeresin. Meanwhile, the “ion exchange ratio of the ion exchange resin” isdetermined by the number of carbons in the polymer backbone and thenumber of cation exchange groups. According to one embodiment of theinvention, the ion exchange ratio ranges from 3 to 33. This correspondsto an equivalent weight (EW) of about 700 to 2000.

In an embodiment, non-limiting examples of the cation exchange resininclude at least one hydrogen ion-conductive polymer selected from thegroup consisting of fluoro-based polymers, benzimidazole-based polymers,polyimide-based polymers, polyetherimide-based polymers,polyphenylenesulfide-based polymers, polysulfone-based polymers,polyethersulfone-based polymers, polyetherketone-based polymers,polyether-etherketone-based polymers, polyphenylquinoxaline-basedpolymers, and combinations thereof.

Specific examples of fluoro-based polymers includepoly(perfluorosulfonic acids) of Formula 1 below ((NAFION®), E. I.Dupont de Nemours Company), Aciplex™ (Asahi Kasei Chemical), Flemion™(Asahi Glass), and Fumion™ (commercialized as Fumatech)), fluorocarbonvinyl ethers of Formula 2 below, or vinyl ether fluorides of Formula 3below. The polymers disclosed in U.S. Pat. Nos. 4,330,654, 4,358,545,4,417,969, 4,610,762, 4,433,082, 5,094,995, 5,596,676, and 4,940,525 mayalso be used, the contents of which are hereby incorporated byreference.

wherein in the above Formula 1, X is H, Li, Na, K, Cs, tetrabutylammonium, or NR1R2R3R4; R1, R2, R3, and R4 are independently selectedfrom H, CH₃, and C₂H₅; m is at least 1; n is more than or equal to 2; xranges from about 3.5 to 5; and y is more than or equal to 1000.MSO₂CFRfCF₂O[CFYCF₂O]_(n)CF=CF₂  (2)

wherein in Formula 2, Rf is fluorine or a C₁ to C₁₀ perfluoroalkylradical; Y is fluorine or a trifluoromethyl radical; n has a value from1 to 3; and M is selected from the group consisting of fluorine, ahydroxyl radical, an amino radical, and —OMe, where Me is an alkalimetal radical or a quaternary ammonium radical.

wherein in Formula 3, k is 0 or 1, and I is an integer ranging from 3 to5.

The above structure represents poly(perfluorosulfonic acid) (productname: NAFION®), but it changes into a micelle structure when a sulfonicacid at the chain end thereof is hydrated. It acts like a typicalaqueous solution acid, providing a passage through which hydrogen ionscan move. When the perfluorosulfonic acid (NAFION®) is used as a cationexchange resin in the invention, in one embodiment X in the ion exchangegroup (—SO₃X) at its side chain end can be replaced with a univalent ionsuch as hydrogen, sodium, potassium, cesium, and the like, andtetrabutylammonium (TBA).

In an embodiment, specific examples of the benzimidazole-based polymers,polyimide-based polymers, polyetherimide-based polymers,polyphenylenesulfide-based polymers, polysulfone-based polymers,polyethersulfone-based polymers, polyetherketone-based polymers,polyether-etherketone-based polymers, or polyphenylquinoxaline-basedpolymers include polybenzimidazole, polyimide, polysulfone, polysulfonederivatives, sulfonated poly (ether ether ketone) (s-PEEK),polyphenyleneoxides, polyphenylenesulfides, polyphosphazenes, and so on.

Alternatively, in one embodiment, an electrolyte where apolystyrenesulfonic acid polymer is grafted on polyethylene,polypropylene polymers, fluoroethylene polymers, orethylene/tetrafluoroethylene polymers may be used.

In one embodiment, a salt of the metal-bound inorganic ion conductor ismixed with an ion-conductive cation exchange resin in a weight ratioranging 1 to 20 based on the ion-conductive cation exchange resin. Whenthe inorganic ion-conductive salt is mixed in a weight ratio of lessthan 1 based on the polymer resin, it cannot suppress the evaporation ofmoisture, thereby failing in maintaining conductivity. Further, whenmixed in an amount of more than 20, the ductility of the polymerelectrolyte membrane increases, leading to the failure of accomplishingmechanical rigidity.

Furthermore, in one embodiment, a polymer electrolyte membrane for afuel cell of the invention can include an inorganic additive. Theinorganic additive is dispersed inside the polymer electrolyte membranewith a predetermined size, further improving mechanical properties. Inan embodiment, the inorganic additive can include fumed silica (productname: Aerosil, Cabo-sil, and so on), clay, alumina, mica, or zeolite(product name: SAPO-5, XSM-5, AIPO-5, VPI-5, MCM-41, and so on).Non-limiting examples of the clay include pyrophylite-talc,montmorillonite (MMT), saponite, fluorohectorite, kaolinite vermiculite,laponite, illite, mica, brittle mica, tetrasilicic mica, and so on.

According to an embodiment of the invention, the inorganic additive isadded in an amount ranging from 1 to 10 parts by weight based on 100parts by weight of the entire polymer electrolyte membrane.

In order to prepare a polymer electrolyte membrane according to oneembodiment of the invention with the aforementioned structure, first, aninorganic ion-conductive salt bound with a non-water-soluble andhydrophilic metal is prepared by reacting a metal compound and aninorganic ion conductor in a solvent. Next, the prepared inorganicion-conductive salt can be produced at higher purity by being fired at300 to 350° C. to decompose unreacted remnants.

In an embodiment, non-limiting examples of the solvent include at leastone selected from the group consisting of water, an alcohol such asmethanol, ethanol, and propanol, N-methyl-2-pyrrolidinone, dimethylformamide, dimethyl acetamide, tetrahydrofuran, dimethyl sulfoxide,acetone, methyl ethyl ketone, tetramethylurea, trimethylphosphate,butyrolactone, isophorone, carbitol acetate, methylisobutylketone,N-butyl acetate, cyclohexanone, diacetone alcohol, diisobutyl ketone,ethylacetoacetate, glycol ether, propylene carbonate, ethylenecarbonate, dimethyl carbonate, diethyl carbonate, and mixtures thereof.

In an embodiment, a metal comprised in the metal compound may includeCs, Na, or K, and the compound can include carbonate, phosphate, orsulfonate.

In one embodiment, the metal compound is mixed with an inorganic ionconductor in a mixing weight ratio ranging 1:3 to 1:6.

Then, in an embodiment, an inorganic ion-conductive salt bound with themetal is mixed with a proton-conductive polymer resin solution, therebypreparing a mixture.

In an embodiment, the proton-conductive polymer resin solution isprepared by adding an ion-conductive cation polymer resin to an organicsolvent. In one embodiment, the solvent can include 1-propanol,2-propanol, ethyl alcohol, methyl alcohol, dimethylacetamide,dimethylformamide, N-methyl-2-pyrrolidinone, and mixtures thereof.

In an embodiment, a mixing ratio of the inorganic ion-conductive saltand an ion-conductive cation polymer resin solution is in the range of 1to 20 based on the polymer resin. In one embodiment, the ion-conductivecation polymer resin solution is in a concentration ranging from 0.5 to30 wt %.

When the polymer resin solution is in a concentration less than 0.5 wt%, the viscosity of the mixing solution becomes lower, leading aninorganic additive with high density to sink during the drying process,consequently resulting in uneven distribution thereof throughout thethickness of the membrane. Meanwhile, when the concentration is morethan 30 wt %, there is the problem of extending the mixing process. Inan embodiment, during the mixing process, an inorganic additive can beadded. In one embodiment, the inorganic additive can be added in anamount ranging from 1 to 20 parts by weight based on 100 parts by weightof the entire mixture.

Then, in an embodiment, the mixture is cast or electrospun to prepare apolymer electrolyte membrane for a fuel cell.

In one embodiment, the casting is performed by coating the mixture on asubstrate such as glass, and then drying it. In an embodiment, thedrying is performed at 100 to 120° C. to evaporate the solvent. When itis dried at a temperature higher than 120° C., the solvent is evaporatedtoo quickly, causing minute cracks on the surface of the electrolytemembrane. In one embodiment, as the solvent is evaporated at apredetermined temperature, the polymer electrolyte membrane is formed ina predetermined thickness, wherein inorganic ion conductors areuniformly distributed in a predetermined size.

According to an embodiment of the invention, a fuel cell system havingthe polymer electrolyte membrane includes at least one electricitygenerating element, a fuel supplier, and an oxidant supplier.

In one embodiment, the electricity generating element includes amembrane-electrode assembly, which includes a polymer electrolytemembrane with a cathode and an anode at respective sides thereof andseparators (referred to as bipolar plates) at respective sides of themembrane-electrode assembly, and it plays a role of generatingelectricity through the oxidation reaction of a fuel and the reductionreaction of an oxidant.

The fuel supplier plays a role of supplying a fuel such as hydrogen tothe electricity generating element, and the oxidant supplier plays arole of supplying an oxidant such as oxygen or air to the electricitygenerating element. In one embodiment, the fuel used in the invention isa hydrogen or hydrocarbon fuel in a liquid state. Examples of thehydrocarbon fuel include methanol, ethanol, propanol, butanol, andnatural gas.

FIG. 1 is the schematic structure of a fuel cell system according to oneembodiment of the invention, which will be described in details withreference to this accompanying drawing as follows. The fuel cell system400 comprises a stack 43 having at least one electricity generatingelement 40 for generating electricity through the electrochemicalreaction of an oxidant, a fuel supplier 4 for supplying the fuel to theelectricity generating element 40, and an oxidant supplier 5 forsupplying the oxidant to the electricity generating element 40.

In addition, in one embodiment, the fuel supplier 4 for supplying thefuel has a fuel tank 45 for storing the fuel.

In one embodiment, the electricity generating element 40 comprises amembrane-electrode assembly 51 for the oxidation/reduction reaction ofthe fuel and oxidant and separators 53 and 55 for supplying airincluding the fuel and oxidant to both sides of the membrane-electrodeassembly 51.

Hereinafter exemplary examples and comparative examples are illustrated.However, it is understood that the following examples do not cover allpossible variations and that the invention is not limited thereto.

EXAMPLE 1

A phosphotungsten acid aqueous solution was prepared by dissolving 12.0g of phosphotungsten acid (P₂O₅.24WO₃.nH₂O, PWA) particles in 50 ml ofdeionized water, and a Cs₂CO₃ aqueous solution was prepared bydissolving 2.5 g of Cs₂CO₃ particles in 50 ml of deionized water. Then,the Cs₂CO₃ aqueous solution was dripped into the phosphotungsten acidaqueous solution with the same pH level for 30 minutes and mixedtogether by using a magnetic agitator to lead a cation substitutedreaction thereof. A Cs⁺-phosphotungsten acid was prepared by heating ofthe final reactant at 300° C. for 2 hours to evaporate the deionizedwater, and additionally by firing it for 2 hours at 350° C. in a furnaceto decompose the remaining non-reactants. The preparedCs⁺-phosphotungsten acid was ball-milled to have a uniform particlesize.

5 wt % of a commercial NAFION®/H₂O/2-propanol (Solution Technology Inc.,EW=1100) solution was evaporated, while being agitated, under reducedpressure at room temperature. Then, the resulting NAFION® was added todimethylacetamide (Aldrich, DMAc) to a 5% concentration, to prepare acation exchange resin solution (5 wt % NAFION®/DMAc) which wasmechanically agitated together at 100° C. for 24 hours.

100 g of the cation exchange resin solution was mixed with 0.5 g of theCs⁺-phosphotungsten acid particles, agitated at 80° C. for 6 hours witha magnetic agitator, and applied with ultrasound at 80° C. for 2 hoursto prepare a uniform polymer mixing solution. The polymer mixingsolution was coated on a glass plate and dried at 100° C. for more than12 hours to evaporate organic solvents, thereby preparing a polymerelectrolyte membrane.

A cathode and an anode were formed by screen-printing a catalystelectrode layer comprising Pt—Ru black (not supported by a carrier,Johnson Matthey, HiSpec 6000) and Pt black (Johnson Matthey, HiSpec1000), which were impregnated with a 5 wt % NAFION®/H₂O/2-propanolsolution on a TEFLON (tetrafluoroethylene) film, drying it, and thenhot-pressing it at 190° C. with a pressure of 200 kgf/cm² for 3 minutesto be loaded in an amount of 4 mg/cm² on the polymer electrolytemembrane.

Next, a membrane-electrode assembly was prepared by positioning andsettling an ELAT electrode substrate (a gas diffusion layer)manufactured by E-Tek Co. at a cathode and anode having the polymerelectrolyte membrane therebetween.

The prepared membrane-electrode assembly was interposed between gaskets,then interposed between two bipolar plates equipped with a gas flowchannel and a cooling channel in a predetermined form, and pressedbetween copper end plates, thereby preparing a unit cell.

EXAMPLE 2

A polymer electrolyte membrane was prepared using the same method as inExample 1, except that a salt of Cs⁺-phosphotungsten acid was preparedby respectively dissolving a phosphotungsten acid and cesium carbonateparticles in 50 ml of ethanol.

EXAMPLE 3

A polymer electrolyte membrane was prepared using the same method as inExample 1, except that a salt of Cs⁺-phosphotungsten acid was preparedby dissolving a phosphotungsten acid and cesium carbonate particles in50 ml of methanol.

EXAMPLE 4

A polymer electrolyte membrane was prepared using the same method as inExample 1, except for using a phosphotungsten acid and sodium carbonate.

EXAMPLE 5

A polymer electrolyte membrane was prepared using the same method as inExample 1, except for using a phosphotungsten acid and calciumcarbonate.

EXAMPLE 6

A polymer electrolyte membrane was prepared using the same method as inExample 1, except for using a silicontungsten acid.

EXAMPLE 7

A polymer electrolyte membrane was prepared using the same method as inExample 1, except for using zirconium hydrogen phosphate.

COMPARATIVE EXAMPLE 1

115 membranes of commercial NAFION® were respectively treated in 3%-hydrogen peroxide and 0.5M of a sulfuric acid aqueous solution at 100°C. for an hour, and then washed in deionized water at 100° C. for 1hour.

COMPARATIVE EXAMPLE 2

A commercial 5 wt %-NAFION®/H₂O/2-propanol (Solution Technology Inc.,EW=1,100) solution was agitated at room temperature and was evaporatedunder reduced pressure. The resulting NAFION® was added toDimethylacetamide (Aldrich) to a 5 wt % concentration and mechanicallyagitated at 100° C. for 24 hours to prepare a cation exchange resinsolution (5 wt % NAFION®/DMAc).

A polymer electrolyte membrane was prepared using the same method as inExample 1, except for adding 0.5 g of a phosphotungsten acid to 100 g ofthe cation exchange resin solution.

SEM Photographs

A SEM photograph of the polymer electrolyte membrane prepared accordingto Comparative Example 2 is provided in FIG. 2. As shown in FIG. 2,coagulations among inorganic conductor particles occurred by stronginteractions in the polymer electrolyte membrane. On the contrary, asshown in a SEM photograph of the polymer electrolyte membrane preparedaccording to Example 1 in FIG. 3, inorganic particles with apredetermined size were uniformly distributed.

Ion Conductivity

Ion conductivity of the polymer electrolyte membranes prepared accordingto the Examples 1 and 2 and Comparative Example 1 was measured by usinga conductivity measurement cell by BekkTech. It was also performed bymeasuring impedances under 100% relative humidity (RH) as thetemperature changed.

The measured conductivities are provided in FIG. 4. As shown in FIG. 4,a fuel cell including the polymer electrolyte membranes prepared byExamples 1 and 2 had stable conductivity compared with ComparativeExample 1 at a cell operation temperature ranging from 50 to 120° C.

Methanol Cross-over

Unit cells prepared according to Example 1 and Comparative Examples 1and 2 were operated by flowing in 4 ml of 1M-methanol and 200 sccm(Standard Cubic Centimeter per Minute, cm³/min) of nitrogen, and themethanol cross-over currents were measured at 60° C. NHE (NormalHydrogen Electrode) of FIG. 5 means a predetermined electrochemicalpotential level.

The results are provided in FIG. 5. As shown in FIG. 5, phosphotungstenacid particles of Comparative Example 2 were dissolved during thepre-treatment process of an electrolyte membrane, so that its methanolcross-over current increased, compared with 115 commercial NAFION®membranes of Comparative Example 1. On the contrary, theCs⁺-phosphotungsten acid salt of Example 1 shows strikingly low methanolcross-over compared with that of Comparative Example 1. As shown in FIG.5, the y axis indicates methanol cross-over currents occurring when themethanol is not crossed over the polymer membrane toward a cathode butis all oxidized at an anode. Accordingly, bigger current values mean agreater amount of methanol cross-over.

*IV Curve

Unit cells prepared according to Example 1 and Comparative Examples 1and 2 were operated by using an air oxidant and an 1M methanol fuel in acondition of 3 of λ (lamda, ratio of real feeding amount of fuel totheoretical feeding amount), and power densities were measured at 70° C.The results are shown in FIG. 6. As shown in FIG. 6, the cell of Example1 exhibits better power density and good battery performances, comparedto Comparative Examples 1 and 2.

As described above, since a polymer electrolyte membrane for a fuel cellof the invention can operate at a high temperature, it can improvereaction speed and also control cross-over of a hydrocarbon fuel whenapplied to a fuel cell.

While this invention has been described in connection with what isconsidered to be exemplary embodiments, it is to be understood that theinvention is not limited to the disclosed embodiments, but, on thecontrary, is intended to cover various modifications and equivalentarrangements included within the spirit and scope of the appended claimsand their equivalents.

1. A polymer electrolyte membrane for a fuel cell, comprising: ametal-bound inorganic ion-conductive salt; and an ion-conductive cationexchange resin, wherein the metal-bound inorganic ion-conductive salt isprovided in the range of from 1 to 20 parts by weight based on 100 partsby weight of the ion-conductive cation exchange resin.
 2. The polymerelectrolyte membrane of claim 1, wherein the metal-bound inorganicion-conductive salt is at least one inorganic ion-conductive saltselected from the group consisting of phosphotungstic acid,silicotungstic acid, zirconium hydrogen phosphate,α-Zr(O_(a1)PCH_(a2)OH)_(a)(O_(b1)PC_(b2)H_(b4)SO_(b5)H)_(b).nH₂O,wherein a1, a2, a, b1, b2, b4, b5, and b are the same or different fromeach other and are integers of 0 to 14, and n is an integer of 0 to 50,ν-Zr(PO_(a1))(H_(a2)PO_(a3))_(a)(HO_(b1)PC_(b2)H_(b3)SO_(b4)H)_(b).nH₂O,wherein a1, a2, a3, a, b1, b2, b3, b4, and b are the same or differentfrom each other and are integers of 0 to 14, and n is an integer of 0 to50, Zr(O_(a1)PC_(a2)H_(a3))_(a)Y_(b), wherein a1, a2, a3, a, and b arethe same or different from each other and are integers of 0 to 14,Zr(O_(a1)PCH_(a2)OH)_(a)Y_(b).nH₂O, wherein a1, a2, a, and b are thesame or different from each other and are integers of 0 to 14, and n isan integer of 0 to 50, α-Zr(O_(a1)PC_(a2)H_(a3)SO_(a4)H)_(a).nH₂O,wherein a1, a2, a3, a4, and a are the same or different from each otherand are integers of 0 to 14, and n is an integer of 0 to 50,α-Zr(O_(a1)POH).H₂O, wherein a1 is an integer of 0 to 14, andcombinations thereof.
 3. The polymer electrolyte membrane of claim 2,wherein the metal-bound inorganic ion-conductive salt is a salt ofCs-bound phosphotungstic acid.
 4. The polymer electrolyte membrane ofclaim 1, wherein the metal is a univalent metal.
 5. The polymerelectrolyte membrane of claim 4, wherein the metal is selected from thegroup consisting of Cs, Na, K, and combinations thereof.
 6. The polymerelectrolyte membrane of claim 1, wherein the ion-conductive cationexchange resin is a polymer resin having a cation exchange groupselected from the groups consisting of sulfonic acid groups, carboxylicacid groups, phosphoric acid groups, phosphonic acid groups, derivativesthereof, and combinations thereof.
 7. The polymer electrolyte membraneof claim 1, wherein the ion-conductive cation exchange resin has anion-exchange ratio in the range of 3 to 33 and an equivalent weight inthe range of 700 to
 2000. 8. The polymer electrolyte membrane of claim1, wherein the ion-conductive cation exchange resin is selected from thegroup consisting of perfluoro-based polymers, benzimidazole-basedpolymers, polyimide-based polymers, polyetherimide-based polymers,polyphenylenesulfide-based polymers, polysulfone-based polymers,polyethersulfone-based polymers, polyetherketone-based polymers,polyether-etherketone-based polymers, polyphenylquinoxaline-basedpolymers, and combinations thereof.
 9. The polymer electrolyte membraneof claim 1, further comprising an inorganic additive.
 10. The polymerelectrolyte membrane of claim 9, wherein the inorganic additive isselected from the group consisting of silica, clay, alumina, mica,zeolite, and combinations thereof.
 11. The polymer electrolyte membraneof claim 9, wherein the inorganic additive is included in an amount inthe range of 1 to 10 parts by weight based on 100 parts by weight of theentire polymer electrolyte membrane.
 12. A fuel cell system comprisingat least one electricity generating element comprising: at least onemembrane-electrode assembly comprising an anode and a cathode positionedon opposite sides of a polymer electrolyte membrane that is interposedtherebetween, wherein the polymer electrolyte membrane comprises ametal-bound inorganic ion-conductive salt and an ion-conductive cationexchange resin, and the metal-bound inorganic ion-conductive salt isprovided in the range of from 1 to 20 parts by weight based on 100 partsby weight of the ion-conductive cation exchange resin; separators onopposite sides of the polymer electrolyte membrane for generatingelectricity through an oxidation reaction of a fuel and a reductionreaction of an oxidant; a fuel supplier for supplying the fuel to theelectricity generating element; and an oxidant supplier for supplyingthe oxidant to the electricity generating element.
 13. The fuel cellsystem of claim 12, wherein the metal-bound inorganic ion-conductivesalt is at least one selected from the group consisting ofphosphotungstic acid, silicotungstic acid, zirconium hydrogen phosphate,α-Zr(O_(a1)PCH_(a2)OH)_(a)(O_(b1)PC_(b2)H_(b4)SO_(b5)H)_(b).nH₂O,wherein a1, a2, a, b1, b2, b4, b5, and b are the same or different fromeach other and are integers of 0 to 14, and n is an integer of 0 to 50,ν-Zr(PO_(a1))(H_(a2)PO_(a3))_(a)(HO_(b1)PC_(b2)H_(b3)SO_(b4)H)_(b).nH₂O,wherein a1, a2, a3, a, b1, b2, b3, b4, and b are the same or differentfrom each other and are integers of 0 to 14, and n is an integer of 0 to50, Zr(O_(a1)PC_(a2)H_(a3))_(a)Y_(b), wherein a1, a2, a3, a, and b arethe same or different from each other and are integers of 0 to 14,Zr(O_(a1)PCH_(a2)OH)_(a)Y_(b).nH₂O, wherein a1, a2, a, and b are thesame or different from each other and are integers of 0 to 14, and n isan integer of 0 to 50, α-Zr(O_(a1)PC_(a2)H_(a3)SO_(a4)H)_(a).nH₂O,wherein a1, a2, a3, a4, and a are the same or different from each otherand are integers of 0 to 14, and n is an integer of 0 to 50 andα-Zr(O_(a1)POH).H₂O, wherein a1 is an integer of 0 to 14, andcombinations thereof.
 14. The fuel cell system of claim 13, wherein themetal-bound inorganic ion-conductive salt is a Cs-bound phosphotungsticacid salt.
 15. The fuel cell system of claim 12, wherein the metal is aunivalent metal.
 16. The fuel cell system of claim 15, wherein the metalis selected from the group consisting of Cs, Na, K, and combinationsthereof.
 17. The fuel cell system of claim 12, wherein theion-conductive cation exchange resin has a cation exchange groupselected from the group consisting of sulfonic acid groups, carboxylicacid groups, phosphoric acid groups, phosphonic acid groups, derivativesthereof, and combinations thereof.
 18. The fuel cell system of claim 12,wherein the ion-conductive cation exchange resin has an ion-exchangeratio in the range of 3 to 33 and an equivalent weight in the range of700 to
 2000. 19. The fuel cell system of claim 12, wherein theion-conductive cation exchange resin is selected from the groupconsisting of perfluoro-based polymers, benzimidazole-based polymers,polyimide-based polymers, polyetherimide-based polymers,polyphenylenesulfide-based polymers, polysulfone-based polymers,polyethersulfone-based polymers, polyetherketone-based polymers,polyether-etherketone-based polymers, polyphenylquinoxaline-basedpolymers, and combinations thereof.
 20. The fuel cell system of claim12, wherein the inorganic ion-conductive salt bound with the metal has aratio in the range of 1 to 20 parts by weight based on 100 parts byweight of the ion-conductive cation exchange resin.
 21. The fuel cellsystem of claim 12, wherein the polymer electrolyte membrane furthercomprises an inorganic additive.
 22. The fuel cell system of claim 21,wherein the inorganic additive is selected from the group consisting ofsilica, clay, alumina, mica, zeolite, and combinations thereof.
 23. Thefuel cell system of claim 21, wherein the inorganic additive is added ina ratio in the range of 1 to 10 parts by weight based on 100 parts byweight of the entire amount of the polymer electrolyte membrane.
 24. Apolymer electrolyte membrane for a fuel cell, comprising: a metal-boundinorganic ion-conductive salt comprising a metal compound and aninorganic ion conductor provided in a weight ratio ranging from 1:3 to1:6; and an ion-conductive cation exchange resin.